Vibration sensors detect free vibrations. In particular, piezoelectric vibration sensors typically include a piezoelectric crystal that generates a current when the crystal is bent during vibrations. The piezoelectric crystal is typically attached to a base of the vibration sensor and another piezoelectric crystal may be attached to the base such that the base is between the two piezoelectric crystals. The current from the one or two piezoelectric crystals during vibration can then be detected to sense vibrations. Machines or machine systems, such as pumps or compressors, occasionally vibrate excessively when a harmful mechanical condition is present. A vibration sensor can detect these vibrations to signal when the machine system should be inspected or serviced for a harmful mechanical condition. These harmful mechanical conditions may be conditions such as machinery failure, unbalance, misalignment, bearing faults, etc. Vibration sensors can be attached to the machine itself or to fittings or other components of the machine system. Different systems in various conditions and situations may produce different vibrations in different frequency ranges, requiring vibration sensors attached to the system to be calibrated to respond appropriately during a desired frequency range that would indicate a harmful mechanical condition for a particular machine system in a particular situation. Therefore the vibration sensor requires a specific resonance frequency that is specifically tailored to fall within the frequency range of the mechanical condition of the specific machine to which the vibration sensor is attached to.